The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue, and more particularly, to an ablation system having multiple-sensor electrodes and a controller for assessing the position of the electrode and the sensors relative to the biological tissue and the adequacy of the energy being applied.
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
During ablation, portions of the electrodes are typically in contact with the blood, so that it is possible for clotting and boiling of blood to occur if those electrodes reach an excessive temperature. Both of these conditions are undesirable. Clotting is particularly troublesome at the surface of the catheter electrode because the impedance at the electrode rises to a level where the power delivery is insufficient to effect ablation. The catheter must be removed and cleaned before the procedure can continue. Additionally, too great a rise in impedance can result in tissue dessication and thrombus formation within the heart, both of which are also undesirable. Further, too great a temperature at the interface between the electrode and the tissue can cause the tissue to reach a high impedance which will attenuate and even block the further transmission of RF energy into the tissue thereby interfering with ablation of tissue at that location.
To avoid these detrimental conditions, RF ablation catheters for cardiac applications typically provide temperature feedback during ablation via a thermal sensor such as a thermocouple. In the case where a catheter has a band electrode, such as for the treatment of atrial fibrillation by the ablation of tissue, the temperature reading provided by a single thermal sensor mounted to the band along the catheter""s outside radius of curvature, may not accurately represent the temperature of the electrode/tissue interface. Typically the side of the band which is in direct contact with the tissue becomes significantly hotter than the rest of the band electrode that is cooled by the blood flow. Thus, the closer the thermal sensor is to the electrode/tissue interface, the more closely the temperature reading provided by the thermal sensor reflects the temperature of the tissue.
The position of the thermal sensor relative to the electrode/tissue interface is influenced by the rotational orientation of the catheter. If the catheter is oriented so that the single thermal sensor is not in contact with the tissue during the application of ablation energy, not only would there be a time lag in the sensor reaching the tissue temperature, but due to the effect of the cooling blood flow, the sensor reading may never approach the actual tissue temperature.
To overcome the effect that the rotational orientation of the catheter has on temperature sensing, two thermal sensors may be used. These thermal sensors are positioned at different locations on the band electrode and are also located about the catheter""s outside radius of curvature, with one electrode being positioned on each side of the radius of curvature. As shown in FIGS. 15 and 16, the outside radius of curvature is the longitudinal line positioned at the outer most point of the outer half of the catheter, most distant from a reference center point of the catheter distal tip curve. Ideally, contact between the catheter and the tissue occurs along this longitudinal line, i.e., the outside radius of curvature. In such catheters it is generally assumed that at least one of the two thermal sensors will be located directly upon the electrode/tissue interface during ablation. Accordingly, while both thermal sensors provide temperature readings, only the highest measured temperature is of clinical interest. This is because the highest sensor reading is expected to reliably represent the electrode/tissue interface temperature.
In a catheter having two thermal sensors, it is still possible that neither sensor is positioned at the electrode/tissue interface. This may occur in situations where the contour of the anatomical structure in which the catheter is placed is such that the band electrode does not contact the tissue. This may also occur where the distal end of the catheter is misoriented such that while tissue contact is made, it is not made along the outside radius of curvature of the catheter. It may also occur where, due to excessive axial twisting of the distal end of the catheter, some of the band electrodes are rotated such that their thermal sensors are no longer oriented about the catheter""s outside radius of curvature. In each of these situations it is possible that the temperature measurements provided by the thermal sensors may not accurately reflect the temperature of the electrode/tissue interface.
In order to provide sufficient electrical energy for effective ablation without unwittingly overheating the electrode/tissue interface and/or forming blood coagulum, it is necessary to first ensure that the electrodes contact the tissue, and second to ensure that the thermal sensors are positioned at or near the electrode/tissue interface so that reliable thermal monitoring of the ablation procedure may occur. For any given band electrode there are several possible scenarios for thermal sensor orientation relative to the electrode/tissue interface. In a first scenario, as shown in FIGS. 18a and 18b, one or both sensors is directly over the electrode/tissue interface. In a second scenario, as shown in FIGS. 19a and 19b, neither sensor is directly over the interface, but their orientation is adequate for one or both to still sense some heating. In a third scenario, as shown in FIGS. 20a and 20b, neither sensor is directly over the interface and their orientation is less than ideal to sense the tissue heating, or else the electrode is not making tissue contact at all (FIG. 20).
Based on the temperature measurement for the electrode it is possible to infer when the third scenario (FIGS. 20a and 20b) is being experienced by a particular band electrode during ablation because there is essentially no temperature response as the applied electrical energy is increased. Unfortunately, with a single temperature value one cannot readily distinguish between the first and second scenarios (FIGS. 18a, 18b and 19a, 19b, respectively) since either exhibits some temperature response with increasing ablation energy.
Hence, those skilled in the art have recognized a need for providing an RF ablation system having a catheter with an electrode carrying multiple thermal sensors for providing temperature readings at a plurality of locations on the electrode and for presenting those readings in a manner which assists in the assessment of both electrode position and thermal sensor position relative to the ablation tissue. The need for automatic control of the energy level applied to an electrode, in view of the electrode and thermal sensor position assessment, has also been recognized. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to an RF ablation system having multiple-sensor electrodes, which in combination with a controller, provide for the assessment of electrode position and thermal sensor position relative to the ablation tissue and assessment/adjustment of energy level application in view of the electrode and thermal sensor positions.
In one aspect, the invention relates to an apparatus for determining the position of a plurality of thermal sensors relative to biological tissue undergoing the application of energy. The apparatus includes a device for commonly carrying the thermal sensors; a catheter for carrying the device and positioning the device proximal the biological tissue; and a processor responsive to the thermal sensors for determining the temperature of each thermal sensor. The apparatus also includes a display responsive to the processor for providing a graphic representation of the temperature of each thermal sensor relative to the temperature of each of the other thermal sensors wherein the graphic representation is indicative of the proximity of the thermal sensors to the biological tissue.
By providing a graphic representation of the temperature of each thermal sensor relative to the temperature of each of the other thermal sensors that is indicative of the proximity of the thermal sensors to the biological tissue, the user is provided with additional information that may aid in deciding whether to adjust the position or orientation of the device relative to the tissue and whether to adjust the applied electrical energy to the catheter""s device during the application of therapy.
In a more detailed aspect, the graphic representation is a bar graph with the temperature of each of the thermal sensors contained within the bar graph. By displaying each of the temperatures in a bar graph, the present invention facilitates rapid visual assimilation of current device/thermal sensor position and orientation. In another aspect, there are two thermal sensors and the graphic representation is a bar graph having the temperature of the first thermal sensor at a first end of the bar graph and the temperature of the second thermal sensor at a second end of the bar graph. In a further facet, the display includes a temperature-range region for displaying an upper temperature range generally indicative of direct contact between at least one of the thermal sensors and the tissue, a middle temperature range generally indicative of near contact between at least one of the thermal sensors and the tissue and a lower temperature range generally indicative of no contact between the thermal sensors and the tissue. The display further includes a device region for displaying a device indicator for each of the devices; and a temperature-data region for displaying a graphic representation which correlates the temperature-range region and the device region to indicate the temperature of the thermal sensors associated with a particular device.
In another facet, the invention is an apparatus for applying energy to biological tissue. The apparatus includes a plurality of electrodes, each with at least one thermal sensor. The apparatus further includes a catheter for carrying the electrodes and positioning the electrodes proximal the biological tissue; a processor responsive to each of the thermal sensors for determining the temperature of the thermal-sensor/electrode junction; and a display responsive to the processor for providing a graphic representation of the temperature of each of the thermal-sensor/electrode junctions relative to the temperature of each of the other thermal-sensor/electrode junctions wherein such representation is indicative of the proximity of each thermal-sensor/electrode junction to the biological tissue.
In a more detailed facet, the plurality of electrodes have at least two thermal sensors and the graphic representation is a bar graph having the temperature of the first thermal sensor at a first end of the bar graph and the temperature of the second thermal sensor at a second end of the bar graph. In another aspect, the length of the bar graph combined with the position of the bar graph relative to the temperature-range region provides an indication of the position of the thermal-sensor/electrode junctions relative to the biological tissue.
In another aspect, the invention is related to an ablation procedure using a catheter having at least one electrode carrying a plurality of thermal sensors and involves a method for determining the position of the thermal sensors relative to the biological tissue undergoing ablation. The method includes monitoring the temperature of each thermal sensor; specifying an upper temperature range generally indicative of direct contact between at least one of the thermal sensors and the tissue; specifying a middle temperature range generally indicative of near contact between at least one of the thermal sensors and the tissue; specifying a lower temperature range generally indicative of no contact between the thermal sensors and the tissue and comparing the monitored temperatures to the temperature ranges.
In a detailed facet, the method also includes, if at least one of the temperatures is within the upper temperature range, maintaining the present position of the electrode relative to the tissue. In another aspect, the method also includes, if each of the temperatures is within the middle temperature range, maintaining the present position of the electrode relative to the tissue. In yet another aspect, the method includes, if each of the temperatures is within the lower temperature range, repositioning the electrode relative to the tissue such that the temperatures are within either one of the middle temperature range or the middle temperature range. In still another facet, comparing the monitored temperatures includes: for each electrode, providing a graphic representation of the temperature of the thermal sensors; and comparing the graphic representation to the upper, middle and lower temperature ranges.
In another facet, the invention is related to an ablation procedure using a catheter having at least one electrode carrying a plurality of thermal sensors, and involves a method of monitoring the application of energy to the biological tissue undergoing ablation. The method includes: (a) positioning the electrode proximal the biological tissue to be ablated; (b) applying a constant energy level to the electrode for a specified period of time; (c) at the end of the specified period of time, determining the temperature of each thermal sensor; (d) displaying the temperature of each thermal sensor; (e) if necessary, adjusting the level of energy applied to the electrode; and (f) if necessary, adjusting the orientation of the electrode relative to the tissue.
In a detailed facet, adjusting the energy level applied to the electrode includes: specifying an upper temperature range generally indicative of direct contact between at least one of the thermal sensors and the tissue; specifying a middle temperature range generally indicative of near contact between the thermal sensors and the tissue; and specifying a lower temperature range generally indicative of no contact between the thermal sensors and the tissue. Adjusting the energy level applied to the electrode further includes: comparing the temperatures of each thermal sensor to the lower, middle and lower temperature ranges; if each of the temperatures is within the upper temperature range, adjusting the energy level such that the temperatures of the thermal sensors are substantially maintained at the temperature necessary to cause ablation; and if each of the temperatures is within the middle temperature range, maintaining the energy level at its present value such that the temperatures of the thermal sensors are maintained within the middle temperature range. In another detailed aspect, adjusting the orientation of the electrodes includes: specifying an upper temperature range generally indicative of direct contact between the thermal sensors and the tissue; specifying a middle temperature range generally indicative of near contact between the thermal sensors and the tissue; specifying a lower temperature range generally indicative of no contact between the thermal sensors and the tissue; comparing the temperatures of each thermal sensor to the upper, middle and lower temperature ranges; and if each of the temperatures is within the lower temperature range, repositioning the electrode relative to the tissue and repeating steps (b) through (f).
In still another aspect, the invention relates to a method of, and an apparatus for, controlling the application of energy to the biological tissue during an ablation procedure using an electrode having at least two thermal sensors attached at separate points. The thermal sensors provide temperature signals indicative of the temperatures of the electrode at the attachment points. The method includes applying power to the electrode while monitoring the spread between the temperatures of the at least two thermal sensors and if the spread exceeds a threshold value, reducing the power. The apparatus includes a generator for applying power to the electrode and a processor programmed to store a threshold value, monitor the spread between the temperatures of the at least two thermal sensors, and cause the generator to reduce the power applied by the generator when the spread exceeds the threshold value.
In yet another aspect, the invention relates to a method of, and an apparatus for, controlling the application of energy to the biological tissue during an ablation procedure. The procedure is done using a catheter comprising a plurality of electrodes. At least two of the electrodes are multiple-sensor electrodes having at least two thermal sensors attached at separate points for providing temperature signals indicative of the temperatures of the electrode at the points of attachment. The method includes, for each multiple-sensor electrode, applying power to the multiple-sensor electrode while monitoring the spread between the temperatures of the at least two thermal sensors associated with the multiple-sensor electrode and if any one of the spreads exceeds a threshold value, at least reducing the power to the multiple-sensor electrode associated with that spread. The apparatus includes a generator operating under the control of a processor to apply power to each multiple-sensor electrode. The processor is programmed to, for each multiple-sensor electrode, monitor the spread between the temperatures of the at least two thermal sensors associated with the multiple-sensor electrode and if any one of the spreads exceeds a threshold value, cause the generator to reduce the power to at least the multiple-sensor electrode associated with that spread.
In another facet, the invention relates to an apparatus for controlling the application of energy to the biological tissue during a tissue ablation procedure using a catheter comprising a plurality of electrodes. At least two of the electrodes have multiple-sensor electrodes having two thermal sensors attached at separate points for providing temperature signals indicative of the temperatures of the electrode at the points of attachment. The apparatus includes a generator operating under the control of a processor to apply power to each multiple-sensor electrode. The processor is programmed to store a target temperature and a spread threshold; for each multiple-sensor electrode, monitor the temperatures of each sensor to first identify those electrodes having at least one temperature that is at least as great as the target temperature for each first identified electrode, monitor the spread between the temperature of the two thermal sensors to second identify those electrodes having a spread less than the spread threshold; compare the power levels of each of the second identified electrodes to third identify the electrode having the lowest power level; and cause the generator to set the power level to each of the multiple-sensor electrodes to a power level substantially equal to the power level of the third identified electrode.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.